The present invention relates to an analyzer for analyzing the composition, structure, or electronic condition of a sample by measuring the kinetic energy of charged particles emitted from the sample and the angular distribution of the particles, and more particularly, to such an analyzer suitable for measuring the energy distribution of the charged particles emitted from the sample or the angular distribution of charged particles of particular energy to be noted from the surface of the sample in a two-dimensional manner.
Conventionally, to analyze the energy of charged particles from a sample, the energy is measured for the charged particles emitted within a particular small solid angle, so that the energy distribution of the charged particles in the direction is examined. The present inventors proposed an improved charged particle analyzer for measuring the angular distribution of the charged paraticles of a given kinetic energy emitted from a sample into a large solid angle, which was issued on Jul. 18, 1989, as a U.S. Pat. No. 4,849,629, entitled "CHARGED PARTICLE ANALYZER". That invention is described with reference to FIG. 1 showing a preferred embodiment of the present invention.
A spherical grid 1 has a center O. Concentric with the spherical grid 1, a spherical electrode 2 is disposed. A sample S is positioned within the spherical grid 1 and far from the spherical center of the spherical grid 1. A screen plate 3 is provided having an opening A symmetric with the sample S, with respect to the center of the spherical grid 1. A two-dimensional detector 4 is positioned within a space opposed to the spherical grid 1 as to the screen plate 3 and faced to the opening A. A suitable voltage is applied between the spherical grid 1 and the spherical electrode 2, then no electric field appears within the bottom space of the spherical grid 1.
When an excitation beam is incident on the sample S, the sample S emits charged particles. The emitted charged particles linealy travel at the bottom space of the spherical grid 1 from the incident portion of the exciting beam. They fly an elliptical orbit having one focus at O within the space between the spherical grid 1 and the spherical electrode 2. Some charged particles with too higher energy are eliminated by being struck with the spherical electrode 2. Some charged particles with lower energy can return to the bottom space of the spherical grid 1. Some charged particles having particular energy defined by a voltage applied between the spherical grid 1 and the spherical electrode 2 can transit through the opening A at the direction parallel with the direction of the emission of the charged particles from the sample S. This means that the charged particles transiting through the opening A all have the same energy, and the angular distribution of the charged particles is the same as the angular distribution of them at the time when they are emitted from the surface of the sample S. The output pictures of the twodimensional detector 4 represent the angular distribution of the charged particles with the particular energy.
The particular energy can be selected by the voltage applied between the spherical grid 1 and the spherical electrode 2.
Thus, the above-described analyzer can measure the angular distribution of the charged particles having the particular energy among the charged particles emitted from the excited portion on the surface of the sample S.
As apparent from the above description, the spherical grid 1 and the spherical electrode 2 do not conform any low pass filter. According to that invention, no energy filter, by which all of the charged particles having energy more than particular energy Ec are introduced into the spherical electrode 2 and all of the charged particles having energy less than energy Ec are reflected, is provided. Otherwise, the charged particles with the particular energy can be gathered into the opening A and transit it while the charged particles with the other energy are scattered over the screen plate 3 not to transit it. The selection of the charged particles in terms of the energy is done in this manner.
The resolution of the energy depends upon the position of the sample, from which the charged particles are emitted, the position and the largeness of the opening A. With a simulation using a computer, the transmittance of the charged particles with various kinetic energies through the opening A is calculated as a function of a deviation of energy as to the particular energy Eo (.DELTA.E/Eo), which results are shown in FIG. 6. The numerals in the graph of FIG. 6 represent 10 times a ratio of a distance s from the center O of the sample S and the opening A as compared to the radius a of the spherical grid 1, namely, 10.times.s/a. When the ratio s/a is small, the transmittance shows that the analyzer is a low-pass filter. When the ratio s/a exceeds 0.5, the transmittance curve is symmetric. When the sample S and the opening A are far from the center O, the half value width of the transmittance is narrow. When the largeness of the opening A is 1% of the radius a of the spherical grid 1 and s/a=0.9, the resolution is about 1%, which is approximate to the resolution of the conventional spectrometer.
By the way, the disadvantage of this spectrometer is that the resolution is not uniform about the emission angle of the charged particles from the sample. Although the total transmittance over all of the emission angles is as illustrated in the graph of FIG. 6, the resolution at some angular region is better than the average value, and at some region it is worse than the average value. In particular, around the condition of emission angle .theta.=90.degree., the resolution is the worst as illustrated by the broken line of FIG. 4. In FIG. 4, the abscissa is the emission angle .theta. and the ordinate is a deviation of energy (%) from the energy (Eo) to be analyzed. In FIG. 4, assuming that the transmittance of the charged particles of the energy Eo is 1, the plotted data represent the energy of the charged particle having about one half of the transmittance. At the range of .theta.=90.degree..about.180.degree., the data are about symmetric with respect to .theta.=90.degree., so that the transmittance of only .theta.=0.degree..about.90.degree. is illustrated. The broken line data of FIG. 4 are given by the previous invention, indicating that around .theta.=90.degree., the resolution is very poor at the high energy side and the low energy side. In practice, around .theta.=90.degree., the S/N ratio is too bad to observe the pictures. In the graph of FIG. 6(b), both sides cannot reach zero promptly due to the poor resolution at this angular range. At the lower angle specified by arrow A in the graph of FIG. 4, the orbit of the charged particles collides with the outer spherical electrode 2, so that the resolution at the high energy side becomes good.